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Abstract. HSX is a toroidal quasihelically-symmetric stellarator with negligibly small toroidal curvature. Vacuum magnetic surfaces at 1 kG are measured using low-energy electron beams that strike a fluorescent mesh. The images are recorded with a CCD camera and show no observable evidence of island structures inside the separatrix. The experimental determination of the rotational transform agrees with numerical calculations to within 1%. A simple analytic expression is derived in Boozer coordinates to relate the drift orbits of passing particles to the magnetic field spectrum. This expression is used to analyze images of high-energy electron orbits in HSX, using a neural network to map the lab coordinate system into Boozer coordinates. At very low magnetic field strengths (90 G) where the b11 component due to the earth s field is not ignorable, this spectral component and the dominant helical term b41 can be experimentally determined. The data does not show the magnitude and direction of the orbit shift that would be expected from the standard toroidal curvature term that exists in other toroidal devices. The results also confirm for the first time that quasihelical stellarators have a large effective transform that is responsible for small drifts of particles off a flux surface.
IAEA 2001
